Garnet films are used as the medium in which magnetostatic waves propagate. Epitaxially grown yttrium iron garnet (YIG) films, for example, exhibit low propagation losses for such magnetostatic waves. Typically, a YIG film is grown on a non-magnetic substrate such as gadolinium gallium garnet (GGG) and exposed to a biasing magnetic field to support the propagation of magnetostatic waves in the film. Magnetostatic waves can be excited by using microstrip structures, while surface acoustic waves can be excited by using an interdigital transducer receiving an rf signal. Acousto-optic Bragg diffractors using surface acoustic wave propagation are limited by SAW technology to operating frequencies of less than 2 GHz.
Garnet film devices are useful for microwave integrated circuit filters due to high Q value resonance characteristics in the microwave frequency band, compact structure, and suitability for mass production by selective patterning processes. In general, garnet film devices have applications for microwave filters, oscillators, instrumentation and communication systems.
Conventionally, as shown in FIG. 1, a microwave channelizer 30 may be comprised of several delay lines 32a, 32b, 32c, . . . forming a filter bank wherein each delay line comprises a separate channel for filtering a signal of particular frequency from a wide band microwave input signal S.sub.i. Each channel includes identically proportioned YIG films which are subjected to a gradient of a magnetic biasing field (not shown). Thus, each delay line is exposed to a magnetic biasing field with a different intensity, and as a result has a different center frequency. The channelizer enables spectral analysis of a microwave input signal by receiving the input signal S.sub.i at each resonator and passing as the output signal S.sub.o from that resonator, the signal component within a narrow band of frequencies about the delay line's center frequency.
In an alternative approach, the delay lines constituting the different channels are replaced by resonators similar to those shown in FIG. 2. The same uniform magnetic field is applied to all of the resonators and center frequency tuning is accomplished by prescribing the dimensions of the YIG film.
Referring to FIG. 2, a typical YIG resonator 10 is shown in which a YIG film 12 of thickness d is epitaxially grown in a non-magnetic GGG substrate 14. Two microstrip lines 16,18 couple microwave energy into and out of the resonator 10 to provide the respective input and output signal paths. The lines 16,18 are provided on a dielectric material 20 of height h, and the YIG film 12 is disposed on that same material which separates the lines 16,18 and the film 12 from a metallic ground plane 22.
Magnetostatic waves typically are generated by passing current through a wire or conductor (such as lines 16 and 18) adjacent to the YIG film. The rf magnetic field surrounding the wire induces MSW propagation in the YIG film. The YIG film functions, in effect, as a waveguide.
A forward surface wave is generated in the film 12 if a magnetic biasing field H is applied in the plane of the YIG film perpendicular to the direction of propagation. A forward volume wave is generated if the magnetic biasing field H is applied normal to the plane of the YIG film 12. A backward volume wave is generated if the magnetic biasing field H is in the plane of the YIG film in the direction of propagation. As illustrated, the magnetic biasing field H.sub.o is applied normal to the plane of the YIG film 12 to cause magnetostatic forward volume waves.